I have an 88 mercruiser 2.5L alpha one. I had it on the water yesterday and it was running really hot, 190+. When i had it mid throttle the temp would drop to 150-160 almost immediatley. Higher throttle and idle would make it overheat. I know it could be anything from waterpump to the thermostat. Question is, the way that it is acting is there a way to narrow it down without replacing everything?